This invention relates to an apparatus for controlling the toner concentration of developer in an electrostatic printing or copying machine.
The latent electrostatic images formed on the drum of a electrophotographic machine are developed by applying a developer consisting of toner and a magnetic carrier. The toner adheres to the latent image whereas the carrier does not so that after repeated use of the developer, the percentage content of the toner is reduced. As a result, it is necessary to supplement the toner so as to maintain its percentage content to obtain accurate copying at a constant image density. This can be done by monitoring the toner concentration within an optimum range. In the prior art, some systems periodically add toner to the developer by utilizing a toner replenishment device. Such systems, however provides constant quantity of toner irrespective of the nature of the documents that have been copied. That is, copying of documents containing substantial black portions require a greater quantity of toner than copying of documents having a lesser amount of black portions. This prior art system, therefore, fails to maintain constant image density under a wide variety of document conditions.
Further, U.S. Pat. No. 3,970,036 (Baer et al., July 20, 1976) discloses a toner concentration detecting and toner replenishing apparatus for use in an electrostatic copying machine. This apparatus comprising a hollow body, a detecting coil wound on the hollow body, and a means for measuring changes in the inductance of the detecting coil. The measuring means includes an oscillator having a tuned circuit which includes the detecting coil as part of the tuned circuit, and circuitry for detecting a change in the frequency of the oscillator. Finally, means are provided which responds to the measuring means for supplying additional toner from a toner supply chamber. Thus prior art apparatus has relatively complicated and costly analog circuitry including a timing circuit, a sensing oscillator, a reference oscillator, a limiter circuit, a detector circuit, a buffer circuit, and various amplifier and transistor stages.